B K Das

Structural and magnetic properties of chemically vapour deposited iron oxide thin films

Abstract Iron oxide thin films have been deposited by a chemical vapour deposition process from a metal-organic iron acetyl acetate source using ArO2 gas mixture transport. As-deposited films are essentially α-Fe2O3. They are transformed by a reduction-oxidati on process to Fe3O4 and γ-Fe2O3 phases as revealed by electron diffraction studies. Microstructural changes and the magnetic properties of the iron oxide thin films as they transform into different states have been studied. These provide new information on the mechanism and transformation kinetics.

Magnetic Properties of Chemical Vapour Deposited Cobalt Dope Gamma Iron Oxide Thin Films

Magnetic properties of chemical vapour deposited (CVD) Fe3O4 and γ-Fe2O3 obtained by co-pyrolysis of ferric and cobaltus acetyl acetonate complexes are described. Saturation magnetization, remanent magnetization and coercivity value of 3.7 kG, 2.2 kG and 820 Oe respectively have been obtained for Co-γ Fe2O3 films. Chemically vapour deposited Co-Fe3O4 films are laterally inhomogeneous with only a fraction of Co2+ ions at octahedral sites (B sites) of spinnel ferrite and excess Co2+ ions distributed in the interstitial matrix. Oxidation conversion to γ-Fe2O3 phase at temperatures above 350°C lead to Co2+ ion diffusion to B sites with activation energy of 0.27 eV. Consequent increase in magneto-crystalline anisotropy results in considerable enhancement in coercivity.

Effect of deposition orientation on the structure and magnetic properties of chemical vapour deposited iron oxide thin films

Abstract Magnetic and structural properties of gamma iron oxide thin films depend significantly on the orientation of the substrate with respect to the direction of source vapour during the growth of films by the reactive chemical vapour dissociation technique. Formation of large grains and grain clusters is observed for flat deposited films as compared with compact small grains at oblique incidence. Such structural variation modifies the anisotropy in the film, resulting in high coercivity of 770 Oe and remanence of 2.7 kG in undoped γ-Fe2O3 films.

The high temperature transition and enhanced Tc (R=0) in Bi1.6Pb0.4Sr2Ca1Cu2O8+x

A high-temperature (HT) superconducting transition around 120 K and enhanced Tc (R=0) were obtained in bulk polycrystalline Bi1.6Pb0.4Sr2Ca1Cu2O8+x samples. In order to understand the physical origin of the HT transition, different studies such as (i) XRD, TEM/SAD, SEM and EDAX and (ii) effects of post-sintering treatments in O2 and Na atmospheres and Ca-excess incorporation have been carried out. From all the structural, microstructural and compositional studies, it appears that the major superconducting bulk consists of Bi-2212 phase without any noticeable amount/intergrowth of the most expected second phase (e.g. Bi-2223) in the sample. Moreover, observed behaviours, which are not expected from the Bi-2223 phase, such as displacement of the HT transition and change in Tc (R=0) value towards (i) high-temperature side on N2 and (ii) lower temperature side on O2 annealing and on Ca-excess incorporation suggest that both the HT transition and enhanced Tc (R=0) are not because of any Bi-2223 phase and on the contrary indicate that they are due to the low-Tc Bi-2212 phase. In addition, the increase in the commensurate modulation periodicity observed from SAD studies suggests that the increase in the order due to an ordered arrangement of oxygen vacancies has resulted in Tc enhancement of the Bi-2212 phase.

Parametric investigation for direct chemical vapour deposition of magnetite films

Abstract A direct growth of magnetite (Fe 3 O 4 ) phase has been obtained by a low-pressure chemical vapour deposition (LPCVD) technique using metal-organic ferric dipivaloylmethanate (Fe(DPM) 3 ) as precursor. On oxidation these films are converted to γ-Fe 2 O 3 phase which give superior magnetic coercivity values. Crystallographic structures are determined by small-angle XRD analysis. As deposited magnetite and oxidized γ-Fe 2 O 3 films are reported with coercivity values of 93 and 122 kA/m, respectively. Squareness ratios of 0.80 are observed for both as-deposited Fe 3 O 4 and oxidized γ-Fe 2 O 3 films with remanent magnetization values of 101 and 102 mT, respectively. Deposition conditions of the as-deposited films along with a growth mechanism involving a gas phase reaction, as promoted by a hot wall type reactor chamber, are described.

Synthesis and characterization of Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O superconductors from the oxine co-precipitation method

Oxide superconductors in the Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O systems with Tc degrees of 93.6 K and 115 K, respectively, have been prepared by the co-precipitation process via the oxine route using nitrates as the starting materials. Chemical homogeneity, stoichiometry and fine particles of the precursor obtained by this process permit a relatively short time for the production of Y- and Bi-based superconducting compounds compared with that obtained by the conventional solid state reaction method. The resulting densities of the sintered samples were typically greater than 90%. X-ray diffraction analysis, energy dispersive spectral analysis and scanning electron microstructural analysis are also reported in this study.

Effect of post deposition reduction treatment of YIG thin films on stabilizing cubic garnet phase

Yttrium iron garnet thin films have been prepared by low pressure metallo-organic chemical vapour deposition method (MOCVD). Dipivaloyl methanates of yttrium and iron have been used as the precursors in the MOCVD growth of the garnet films. Post deposition O2 annealing at 900°C is required to form a garnet phase which also shows orthoferrite and component Y2O3 and α-Fe2O3 phases. We show that a partial H2 reduction treatment minimizes secondary phases and stabilizes the garnet phase. These treatments also enhance the magnetic properties considerably.

Growth, magnetic properties and the effect of cobalt addition in metal-organic chemical vapour deposited (MOCVD) gamma iron oxide thin films for magnetic recording applications

The present study aims at investigating MOCVD technique for the deposition of magnetic oxide thin films using volatile metal-organic compounds as source material. A three-step scheme has been described to form γ-Fe2O3 phase starting from α-Fe2O3 films as-deposited in optically heated atmospheric cold wall CVD reactor. Growth of γ-Fe2O3 in a two-step process has been performed by depositing Fe3O4 phase directly by resistively heated low-pressure CVD (LPCVD) technique. Role of substrate temperature in controlling the oxidation leading to direct formation of metastable γ-Fe2O3 phase (single-step scheme) by atmospheric CVD technique has been described. A new mode of introduction of cobalt in the film, namely heterogeneous dispersion of cobalt in the γ-Fe2O3 matrix, has also been described. Crystallographic structure, microstructure and magnetic properties of the films have been studied in detail. Biaxial vector coil and high-temperature magnetic studies were carried out for determining the nature of anisotropy in the γ-Fe2O3 film. Growth of γ-Fe2O3 films in different schemes have been discussed from the studies of growth kinetics in a cold-and hot-wall-type reactor chambers.

Flux dynamics of YBCO/Ag composite

DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andHmax, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJcgm on low-field M-H loop. The estimation of intergranular critical current densityJcjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJc(T) with silver content under low AC fields only, consistent with the transportJc data; beyond thatJc(T) decreases. This improvement inJc(T) and transportJc with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (Hmax>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionHc2j and lower critical field of intragrain regionHc1g also decrease with silver content.

Effect of Electromagnetic Interactions on the Undulatory Temperature Dependent Behaviour of Non-Resonant Microwave Absorption Signal Amplitude in Bi2Sr2CaCu2O8+x

c � 96 K) while increasing temperature from 15 to 105 K has been investigated and discussed. These studies show an undulatory behavior of an initial fall in the amplitude (15 – 71.8 K) with a subsequent narrow weak temperature independent region (71.8 – 75.6 K) and then a rise peaking at � 82:5 K followed by a final exponential fall (82.5 –105 K). A detailed discussion on earlier reported data has suggested that this undulatory behavior cannot be understood in terms of existing approaches involving effect of Josephson interactions (JI) alone among vortices. In our opinion, the entire undulatory behavior observed in the present samples can be explained with the inclusion of electromagnetic interaction (EMI) along with JI. [DOI: 10.1143/JJAP.45.84]